The present invention relates to ensuring bandwidth availability in a communications system, and more particularly to ensuring such bandwidth availability in a communications system. Even more particularly, the present invention relates to ensuring bandwidth availability in a point-to-point messaging portion of a digital control channel in a communications system.
Interim Standard (IS) 136 (promulgated by the Telecommunications Industry Association) adds a Digital Control Channel (DCCH) to IS-54B, an 800 MHZ TDMA Cellular Standard. The fundamental unit of measure in the digital control channel is a slot. Slots are logically combined (in groups of 16 for a half-rate digital control channel and 32 for a full-rate digital control channel) to form "Superframes". Of the slots in a Superframe available for signaling, some are designated by the base station (BMI) for broadcast (point-to-multipoint) messaging and the rest for point-to-point messaging. In order for the base station to be able to notify (or "page") a mobile station (MS) of an incoming call (or other impending transaction), the mobile station is assigned to one and only one of the slots in a Superframe available for point-to-point messaging on the forward digital control channel (i.e., that portion of the digital control channel used to transmit messages from the base station to the mobile station). Note that the term mobile station is used herein to refer to a radio unit in a communications system, and is not limited to being "mobile." The term "mobile" unit is used herein merely because of its wide (and perhaps unfortunate) acceptance and clear meaning in the communications arts, which includes radio units that are "stationary" or "fixed". When the base station needs to "page" the mobile station, i.e., notify the mobile station that it has an incoming call (or other impending transaction), the base station transmits a "page message" or a "hard page" in the assigned slot. (An advantageous approach to utilizing "hard pages" is described in U.S. patent pending application Ser. No. 08/394,091, entitled OPTIMAL PAGING OF ONE OR TWO MOBILE STATIONS USING A HARD PAGE SLOT, commonly invented and assigned with the present patent document, incorporated herein by reference.) Under quiescent conditions, the mobile station need only monitor this assigned slot in the Superframe. Thus, the mobile station is able to "sleep" while the other 31 slots of the Superframe are being transmitted. In addition, because every other Superframe transmitted (i.e., every primary Superframe) by the base station is followed by a Superframe (secondary Superframe) having identical point-to-point paging slots, the mobile unit can sleep during every other entire Superframe. For other types of point-to-point traffic, however, the mobile station is not assigned to a specific slot in a Superframe; rather, when it is expecting a "non-page message," i.e., not a message intended to notify the mobile station of an incoming call (or other impending transaction), from the base station, the mobile station is required to search for non-page messages addressed to itself in each slot available for point-to-point messaging within the Superframe.
Currently, mobile stations are hashed to a slot in the Superframe (called a PCH Subchannel) where the mobile station it expects to receive page traffic. Nominally, the mobile station is required to read this same slot in every other Superframe.
For illustration purposes, assume a minimal full-rate digital control channel containing four broadcast control channel (BCCH) slots and no Reserved slots. Hence, there are 28 point-to-point message slots, i.e., point-to-point messaging channel slots available for paging (i.e., available for use as PCH Subchannels.) If only 10% of the UPR maximum paging capacity (i.e., User Performance Requirements maximum paging capacity, which is defined as 373,000 pages/hour, or 14 pages/hyperframe by C.T.I.A.) is assumed, then up to one-half of the available point-to-point messaging channel bandwidth could be dedicated to paging. As either the paging traffic increases or the number of point-to-point messaging slots decrease (due to, e.g., additional BCCH messaging), the ratio of pages to PCH Subchannels can approach or exceed 1:1.
The consequence of an increase in paging traffic or corresponding decrease in point-to-point messaging channel bandwidth is that the number of slots available for other messages (e.g., non-page messages) is reduced, and consecutive slots for multi-slot messages become scarce. (The term multi-slot message, as used herein, refers to a page message or a non-page message that spans more than one point-to-point messaging channel slot.) Thus throughput of multi-slot messages and non-page messages may be decreased as paging traffic increases, causing delay in the transmission of such messages.
As used herein, the terms "page message" and "hard page" refers to one or more slots of data transmitted from a base station over the point-to-point messaging channel that contains information intended to signal one (or possibly more) of a plurality of transceiver units that such transceiver unit(s) have an incoming call. (An incoming call can be, e.g., a voice call or any other type of incoming call capable of being serviced by the base station and transceiver units.) The term "non-page message" refers to any slots of data transmitted over the point-to-point messaging channel that are not "page messages" or "hard pages."
Since IS-136 provides for multi-page PCH frames (meaning that more than one mobile station may be paged in a single PCH Subchannel), the volume of available PCH Subchannels, i.e., point-to-point messaging slots assignable as PCH Subchannels, is not a primary issue. However, to compensate for the decrease in bandwidth available for single and multi-slot non-page point-to-point message traffic, the base station must increasingly rely on a technique known as "PCH Continuation" to open up gaps into which this non-page point-to-point message traffic can be accommodated. In a worst case scenario, pages must be delayed, i.e., displaced into later slots, in order to allow some minimal bandwidth for non-page point-to-point message traffic. Such delay, when it affects pages destined for PCH Subchannels late in the primary Superframe, may result in such pages being delayed into a subsequent primary Superframe.
PCH Continuation (PCON) is the process by which the base station directs a mobile station to continue reading a number of point-to-point messaging slots after it first reads its assigned PCH Subchannel. Whenever a mobile station reads its assigned PCH Subchannel and determines that there is no message addressed to its mobile station identification number (MSID), it reads a PCON bit that is carried in the PCH Subchannel. If the PCON bit is set to 0, for example, the mobile station may sleep until the next occurrence of its assigned PCH Subchannel in the next primary Superframe. On the other hand, when the base station sets PCON equal to 1, i.e., when the base station activates PCH Continuation, the mobile station responds by reading additional point-to-point messaging slots, as determined by a PCH.sub.-- DISPLACEMENT parameter sent on the BCCH.
When PCH Continuation is activated on a full-rate digital control channel, the mobile station reads every other point-to-point messaging slot after its assigned PCH Subchannel until a number of additional point-to-point messaging slots equal to the PCH.sub.-- DISPLACEMENT parameter have been read or until it reads a page message addressed to its mobile station identification (MSID) number. When a mobile station has read the last point-to-point messaging slot in the primary Superframe and the number of slots equal to the PCH.sub.-- DISPLACEMENT parameter still have not yet been read, the mobile station continues reading slots beginning with the second point-to-point messaging channel slot in the next primary Superframe. Similarly, if a mobile station has read the second to last point-to-point slot in a primary Superframe and the number of slots equal to the PCH.sub.-- DISPLACEMENT parameter still have not been read, the mobile station continues reading slots beginning with the first point-to-point messaging slot in the next primary Superframe.
When PCH Continuation is activated on a half-rate digital control channel, the mobile station operates as it does on a full-rate digital control channel, except that instead of reading every other slot in the point-to-point messaging channel, it reads every point-to-point messaging slot after its assigned PCH Subchannel. The reading of every point-to-point messaging slot continues until the number of slots equal to the PCH.sub.-- DISPLACEMENT parameter has been read or until the mobile station receives a page message addressed to its mobile station identification (MSID).
Point-to-point message continuation is the process by which the base station sends a message over multiple point-to-point messaging slots. A mobile station responds to point-to-point messaging channel message continuation whenever it determines that a message addressed to its mobile station identification (MSID) number, sent by the base station, occupies more than one point-to-point messaging slot. A page message spanning more than 1 point-to-point messaging slot, when sent on a full-rate digital control channel, is transmitted using every other point-to-point messaging slot. A page message spanning more than 1 point-to-point messaging slot, when sent on a half-rate digital control channel, is transmitted using consecutive point-to-point messaging slots. A non-page message spanning more than 1 point-to-point messaging slot is transmitted using consecutive point-to-point messaging slots unless interrupted.